1. Field of the Invention
This invention relates to an image reading lens, and particularly to an image reading lens suitable for an apparatus such as a facsimile apparatus or an image scanner for causing image information on the surface of an original, which is illuminated by a light beam from illuminating means such as an LED array, to be formed on the surface of reading means such as an image sensor (CCD), and reading the image information.
2. Related Background Art
There have heretofore been proposed various apparatuses designed to cause image information on the surface of an original to be imaged on the surface of a solid state image pickup element array (line sensor) at a predetermined magnification by an image reading lens and then to read the image information. For example, as an apparatus designed to read image information by the use of an image reading lens having a resolving power corresponding to a scanning line density of 8 pel (pels/mm) or 200 dpi (dot/inch), there is an apparatus such as a facsimile apparatus or an image scanner.
An image reading lens having, for example, F number 5.6 and a half angle of view of the order of 22.degree. has heretofore been used in such an apparatus.
Such an image reading lens is proposed, for example, in Japanese Laid-Open Patent Application No. 1-128028. In this publication, the image reading lens is comprised of a so-called triplet type lens comprising three lenses of predetermined shapes, i.e., in succession from the object side, a positive lens, a negative lens and a positive lens, and an aspherical surface is adopted as the image plane side lens surface of the negative lens to thereby achieve an improved optical performance.
Such an image reading lens as a whole is comprised of three lenses of predetermined shapes, and uses glass of high refractive index as the material of the lenses. An aspherical surface is adopted in one of the three lenses. This has led to the problem that the lens construction becomes complicated and relatively high assembly accuracy is required.
Now, in recent years, the tendency of a CCD (line sensor), which is a light receiving element, has advanced toward having a higher sensitivity and along therewith, the tendency of illuminating means (a light source) for illuminating an original (image) has advanced toward having a lower luminance.
As an example of this, there have been proposed various image reading apparatuses using as illuminating means an LED array comprising a plurality of LEDs (light emitting diodes) of simple construction and low cost arranged side by side, and utilizing this LED array to illuminate an image.
The spectral energy distribution of this LED array used as the illuminating means concentrates in a very narrow wavelength area. Therefore, it is important for an image reading lens, for reading image information on the surface of an original illuminated by a light beam from the illuminating means comprising such LED array, to correct various aberrations. In particular, the chromatic aberration of magnification must be corrected in a particularly narrow wavelength area, even better than in a conventional image reading lens that is directed to wide band spectral energy.